Arguably, the whole situation of the Pit and the Pendulum is extreme in its entirety, as it is about death by torture. But within that, there is one situation that sticks out in how extreme it is: the narrator's escape from the pendulum. The narrator can only watch as the pendulum swings lower and lower with more and more force, making its way closer to him agonizingly slowly. He tries to escape on his own, but when he realizes that he cannot do that, he lures the rats onto him and, by spreading the last of his food upon the bonds that hold him down, makes them chew through the only thing holding him in place. The suspense of whether or not he will escape is heightened when he feels the blade actually cut his skin, but then the rats finally break all the bonds, and when the pendulum has passed him, he manages to move out of the way.
The setting of this story is also extreme in its entirety, but perhaps most of all when the walls start closing in on him - literally. He describes the characters on the wall as having glowing, demon eyes that stare him down, and the images of blood are all very vibrant. The walls start closing in on him in the shape of a diamond, forcing him towards the pit, and the only reason he escapes is because the French arrived to take over the Inquisition and they saved him. (That is by no means historically accurate - in fact, none of this is really - but that is what Poe decided had happened).